This invention relates generally to optical materials for, and to a method of, forming optical memory discs of the CD-ROM type for use with fluorescent reading, including disks for three-dimensional (3D) optical memory systems.
To date, digital information carriers for recording, storing, and reading sound and images by optical methods have received wide recognition.
Generally, information is written in the form of local changes to the active medium optical thickness or the reflection coefficient, while reading is performed using laser emission and is based on the laser beam phase or amplitude changes in information record centers.
CD-ROM""s are the least expensive and most sophisticated of the optical information carriers. However, both the storage volume and the signal-to-noise ratio of currently used CD-ROM""s are inadequate for the new generations of computers and video systems now under development.
Therefore, materials for better optical memory systems are being actively developed. These materials are required to provide an increased data density, a high signal-to-noise ratio, an improved operation and storing stability, and low costs.
A promising approach to the increase of optical information carrier capacity consists of increasing the number of information bits per active layer by shortening the pits and increasing their amount, and producing multilayer disks.
This approach is implemented in the DVD standard disks that have recently appeared on the market. In these disks, the pit size is halved and the number of active layers is increased to 4, two on each side of the substrate. This has enabled the disk capacity to be increased up to 20 GB.
However, when reflection coefficient changes are used for reading, further increase of the number of disk active layers results in a sharp cost increase of the system and deterioration of data retrieval quality. As a result, attempts to increase the disk capacity fail.
Known in the prior art are Japanese patent publications [JP 63-195,838 (Aug. 12, 1988); JP 02-308,439 (Dec. 21, 1990)] suggesting a fluorescent reading method. The concept of this method is that after the writing process the information record centers do not fluoresce, whereas the background does. When reading with a corresponding laser beam, the fluorescent light is excited and registered with a detector.
The above references suggest laser beam recorded optical disks which are of the WORM (write once, read many) type and thus cannot be used as CD-ROM type disks. Additionally, the optical disks proposed in the first of the above references are so constructed that the active layer is deposited on a dull substrate surface. This technology prevents the production of multilayer disks, because light excited on reading is subject to strong scattering.
Materials based on photochromic compounds, preferably spirobenzopyrans, contained in a polymer matrix, were suggested by D. A. Pathenopulos, and R. M. Rentzepis, Science, 245, 843 (1989); and R. M. Rentzepis, U.S. Pat. No. 5,268,862 (Dec. 7, 1993) to be used as 3D optical memory medium where multilayer data writing and fluorescent reading are performed. With these materials, information is written by the two-photon excitation of the initial non-fluorescing form A of the photochromic compound, this excitation being produced by two focused laser beams intersecting in definite locations of active medium.
Under excitation, form A transforms to form B. Form B absorbs two reading emission photons and emits the fluorescent light which is absorbed by a detector. Materials of this type are intended for repeatedly rewritten memory, because heating or irradiation results in erasing data due to the transformation of form B to initial form A.
A practical implementation of 3D optical memory concepts and methods, as suggested by U.S. Pat. No. 5,268,862, is quite problematic and highly improbable for a number of reasons, including, in particular, the following:
A low photochemical and thermal stability of suggested photochromic compounds resulting in their destruction after repeated writingxe2x80x94readingxe2x80x94erasing cycles.
The possibility of erasing data (form B to form A transition) in the process of fluorescence reading.
A low quantum yield of merocyanine form B of suggested photochromic spiropyrans.
For the above reasons, the 3D optical memory devices proposed in U.S. Pat. No. 5,268,862 are unsuitable for CD ROM type multilayer disks as well.
The use of organic dyes in optic WORM disks is known D. J. Gravesteijn, J. van der Veen, Philips Tech. Rev., 41 (1983/1984), 325; J. E. Kuder, J. Imag. Technol., 12 (1986), 140; J. E. Kuder, J. Imag. Sci., 32 (1988), 51.
The most frequently used to produce WORM disks active layers are cyanine dyes in JP 08-108,630 (Apr. 30, 1996); JP 07-186,530 (Jul. 25, 1995); JP 06-336,086 (Dec. 6, 1994); JP 06-227,138 (Aug. 16, 1994); phthalocyanines Brit. UK Pat. Appl. GB 2,290,489 (Jan. 3, 1996); JP 07-166,082 (Jun. 27, 1995); Ger. Offen. DE 4,310,917 (Oct. 7, 1993) and porphyrins JP 08-127,174 (May 21, 1996); JP 07-304,257 (Nov. 21, 1995) herein incorporated by reference.
The majority of dyes belonging to these classes exhibit satisfactory fluorescence capacity in polymer matrices provided that true solutions have been formed, i.e. dyes are present in the polymer in a molecularly dispersed state.
In existing WORM disk technology, dyes are usually applied to a grooved substrate by spin coating or another method, e.g. vacuum evaporation, in the form of polycrystalline or amorphous opaque thin films having a thickness of about 10 nm. For this reason, and due to the fact that the laser wavelength being used for reading by the reflection coefficient variation method may not coincide with the dye absorption maximum, the dye concentration of the active layer should be as high as possible. In such layers the dyes used do not fluoresce. In some cases, to obtain dye layers for WORM disks, polymer binders are used, M. S. Gupta, xe2x80x9cLaser Recording on an Overcoated Organic Dye-Binder Mediumxe2x80x9d, Applied Optics, 23(22), Nov. 15, 1984; K. Y. Law et al., xe2x80x9cAblative Optical Recording Using Organic Dye-in-Polymer Filmsxe2x80x9d, Appl. Phys. Lett., 36(11), Jun. 1, 1980; U.S. Pat. No. 5,348,841 (Sep. 20, 1994); JP 07-126,559 (May 16, 1995). In dye-in-polymer films of WORM disks, dyes are only partially dissolved in the polymer matrix, the major part of the dyes staying in the form of aggregates and solid particles. This sharply reduces their fluorescence capacity.
Therefore, dye layers, both with and without a polymer binder, used in WORM disks with the reading based on the variation of reflection coefficient, cannot be used in optical CD-ROM type memory disks with a fluorescence reading.
It is an object of the invention to provide a fluorescence-based reading method that has a number of advantages over methods based on the variation of reflection coefficient, even in the case of single-layer disk.
It is another object of the invention to reduce the precision requirements to pit production as compared to existing CD-ROM""s. For example, changing the pit size by 100 nm does not hinder reading from the fluorescence disk, while resulting in the complete disappearance of the reflection disk signal.
It is a further object of the invention to provide fluorescence disk insensitivity to disk plane tilt variations up to 1xc2x0, which is absolutely inadmissible for the reflection disk.
It is a still further object of the invention to provide a 3D optical memory carrier in the form of a multilayer disk.
To achieve these and other objects, the construction principle of a multilayer optical disk with fluorescence-based reading is as follows B. Glushko, U.S. Provisional Patent Application No. 25457, Aug. 5, 1997 herein incorporated by reference. Single-layer optical disks with pit-filling fluorescent material as information carrier are sequentially superimposed on one another so that a multilayer system is formed, where active layers consisting of fluorescing pits 0.5 to 1 xcexcm deep alternating with inactive separation layers 20 to 50 xcexcm thick, the latter being transparent for both the excitation laser light and the fluorescence light.
To produce a multilayer optical disk with fluorescence-based reading, a fluorescent composition meeting a number of specific requirements is used. The most important of these requirements are as follows:
1. The absorption bandwidth of the fluorescent compound should coincide with the emission wavelength of a laser used for reading.
2. The material should have the maximum possible quantum yield of fluorescence which will not decrease in long-term storage and operation.
3. To avoid over-absorption of fluorescence light, the overlay of the absorption and fluorescence bands should not be too large.
4. The fluorescent composition should not scatter the excitation laser emission and fluorescent light passing through it.
5. The fluorescent dye should be readily compatible with a matrix, forming a true solution of the needed concentration in the matrix, and should not migrate from it.
6. The fluorescent composition should penetrate the pits easily and not color space among the pits.
7. The solution used for filling pits must not interact with a pit-containing substrate and change the geometric parameters of the pits.
8. The refraction index of the fluorescent composition must be close to that of a pit-containing substrate.
9. When storing and using a multilayer disk with pits that contain fluorescent composition, the properties of the disk must not change.
Still another object of the present invention is to develop a fluorescent composition exhibiting physical and physicochemical properties needed for use in CD-ROM type optical disks, including those intended for a 3D optical memory with fluorescence reading, where data carriers represent pits filled with fluorescent composition.
This fluorescent composition consisting of fluorescent dye, film-forming polymer, organic solvent, plastifier and (if necessary) surfactant and light stabilizer provides forming active layers of CD-ROM""s with fluorescence reading, free from the drawbacks of prior art optical disks with active layers based on polymer films containing organic dyes.
This object is achieved by devising a composition including:
A fluorescent dye whose absorption band coincides with the wavelength of the laser emission used for reading, this dye having a high quantum yield of fluorescence, forming a true solution with a polymer matrix and not migrating from the matrix;
A film-forming polymer which exhibits a high transparency and does not scatter the laser emission and fluorescent light passing through the active layer, penetrates the substrate pits well and can easily be removed from the substrate;
An organic solvent that provides good solubility of the fluorescent composition components, wets the pit-containing substrate surface, does not interact with substrate material and does not produce deformation of pits;
A plastisizer which increases the elasticity of composition and helping its penetration into the pits.
Also, if necessary, the composition may be completed with surfactants that reduce the surface tension of the composition and thus improve substrate surface wetting and penetration of the solution into the pits; and with light-stabilizers, contributing to the preservation of optical and spectral luminescence properties of the fluorescent composition in the storage and use of optical disks.
After being applied to the substrate and dried, the fluorescent composition has a refractive index close to that of substrate.